The present invention relates to a process for the gas-phase polymerization of olefins using a reactor having interconnected polymerisation zones. In particular, the invention relates to a gas-phase polymerisation carried out in a reactor having interconnected polymerisation zones, where the first polymerization zone is operated in a “condensed mode”.
The development of catalysts with high activity and selectivity of the Ziegler-Natta type and, more recently, of the metallocene type has led to the widespread use on an industrial scale of processes in which the olefin polymerization is carried out in a gaseous medium in the presence of a solid catalyst. The polymerization of olefins is an exothermic reaction and it is therefore necessary to provide means to cool the bed to remove the heat of polymerization. In the absence of such cooling the bed would increase in temperature until, for example, the catalyst turns inactive or the polymer particles are partially fused. In a fluidized bed polymerization, the preferred method for removing the heat of polymerization is by feeding to the polymerization reactor a recycle gas stream at a temperature lower than the desired polymerization temperature. Such a gas stream, by passing through the fluidized bed, allows conducting away the heat of polymerization. The recycle gas stream is withdrawn from the upper zone of the reactor, cooled by passage through an external heat exchanger and then recycled to the reactor. The temperature of the recycle gas can be adjusted in the heat exchanger to maintain the fluidized bed at the desired polymerization temperature.
EP 089691 relates to a process for increasing the space time yield in a continuous gas fluidized bed process for the polymerization of olefins. According to this patent, the recycle gas stream is intentionally cooled to a temperature below the dew point of the recycle gas stream to produce a two-phase gas/liquid mixture under conditions such that the liquid phase of said mixture will remain entrained in the gas phase of said mixture. The heat of polymerization is removed by introducing said two-phase mixture into the reactor at a point in the lower region of the reactor, and most preferably at the bottom of the reactor to ensure uniformity of the fluid stream passing upwardly through the fluidized bed. The evaporation of the liquid phase takes place inside the polymerization bed and this ensures a more effective removal of the heat of polymerization. This technique is referred to as operation in the “condensing mode”. By operating in the “condensing mode”, the cooling capacity of the recycle stream is increased by both the vaporization of the condensed liquids entrained in the recycle stream and as a result of the greater temperature gradient between the entering recycle stream and the reactor.
EP 699 213 relates to a continuous fluidized bed process for the polymerization of olefins operating in the condensing mode. According to this patent, after the cooling of the recycle stream at a temperature below its dew point, at least part of the condensed liquid is separated by the gas phase and introduced directly into the fluidized bed. In order to gain the maximum benefit in term of cooling of the fluidized bed, the separated liquid must be introduced in the region of the bed that has substantially reached the temperature of the gaseous stream leaving the reactor. The introduction of the separated liquid may be carried out at a plurality of points within this region of the fluidized bed, and these points may be at different heights within this region. Injection means are required, preferably nozzles, arranged such that they protrude substantially vertically into the fluidized bed or may be arranged such that they protrude from the walls of the reactor in a substantially horizontal direction.
A novel gas-phase polymerization process, which represents a gas-phase technology alternative to the fluidized bed reactor technology, as to the preparation of olefin polymers, is disclosed in EP-B-1012195. The polymerization process is carried out in a gas-phase reactor having interconnected polymerization zones, where the growing polymer particles flow through a first polymerization zone (riser) under fast fluidization or transport conditions, leave said riser and enter a second polymerization zone (downcomer) through which they flow in a densified form under the action of gravity, leave said downcomer and are reintroduced into the riser, thus establishing a circulation of polymer between the two polymerization zones.
This polymerization process allows to obtain polymers with a broad molecular weight distribution by establishing different polymerisation conditions in the two interconnected polymerisation zones. This is achieved by introducing into the upper part of the downcomer a gas/liquid mixture, which evaporates and forms a barrier stream preventing or limiting the gases present in the riser from entering the downcomer. Accordingly, different polymerisation conditions can be maintained in the riser and in the downcomer.
The specification of EP 1012195 only mentions that the gas recycle stream entering the riser may contain entrained droplets of liquid, as it is customary when operating in the so-called “condensing mode”. However, this document totally fails to point out any criticality in the step of cooling the gas recycle stream below its dew point. Furthermore, all the polymerization runs in the working examples of this patent are performed avoiding any condensation of the gas recycle stream, which continuously flows upwards along the riser and ensures the transport of the polymer particles in this polymerization zone.
Moreover, a high-power compressor must be always arranged on the recycle line to recycle the gas stream with a pressure and velocity suitable to ensure fast fluidization conditions in the riser: this high-power compressor causes a high energy consumption and increases the operating costs of the polymerization plant.
In view of the above, it would be desirable to decrease the power consumption required by the compressor of the recycle line, while improving the heat removal inside the first polymerization zone (riser).